Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the pyrolysis furnace for further downstream processing, including quenching.
The use of gas turbines (GT) to produce power while simultaneously utilizing the hot, oxygen rich exhaust gas as combustion air in the cracking furnaces is a very attractive means of reducing energy requirements per unit of ethylene production. Such process is known and is explained e.g. in Integrating Gas Turbines with Cracking Heaters in Ethylene Plants, International Journal of Engineering Research & Technology (IJERT), Vol. 3 Issue 6, June 2014, p. 820-825. The use of turbine exhaust gas (TEG) is an effective means providing high level air preheat lowering the heater's fuel requirement. However, unlike air preheat systems, because of the reduced oxygen content of TEG, the total mass flow of flue gas passing through the furnaces is increased. This results in increased steam production in the convection (heat recovery) section of the pyrolysis module.
WO2015128035 discloses a process for increasing process furnaces energy efficiency through gas turbine integration by using turbine exhaust gas. The exhaust gas is fed to a heat recovery unit. The system of WO2015128035 has a problem that the use of preheated air results in higher flame temperatures and the formation of higher amounts of undesired NOx after combustion in the flue gas.
According to its abstract, US 2006/0080970 describes a method of heating a fluid in a fluid heater integrated with a gas turbine used in connection with a steam methane reformer requiring a catalyst.
There is a demand for a process which is energy efficient and in which the amount of NOx is reduced. There is also a demand for a method for increasing the energy efficiency that saves fuel burned by the furnace rather than increasing the steam production by the furnace.